The present invention relates generally, as indicated, to anti-skid brake control systems and, more particularly, to an adaptive acceleration system therefor. The adaptive acceleration system adapts to the spin up or acceleration of the vehicle wheels and correspondingly determines and controls the reapplication of an appropriate amount of brake pressure after a pressure dump, for example, by controlling the amount of air pressure being delivered to the vehicle brake system.
Anti-skid brake control systems are shown, for example, in U.S. Pat. Nos. 3,917,359 and 4,040,676. Anti-skid brake control systems also are shown in copending U.S. patent application Ser. No. 769,255, filed Feb. 16, 1977, for "Anti-Skid Brake Control System With Power-Up Delay" and in copending U.S. patent application Ser. No. 770,535, filed Feb. 22, 1977, now U.S. Pat. No. 4,090,741, for "Anti-Skid Brake Control System With Circuit For Monitoring Slower Wheel", with both of these copending applications being commonly assigned with the present application. The disclosures of these patents and applications are incorporated by reference.
It is the principal purpose of an anti-skid brake control system to provide automatic overriding control of the brakes of a vehicle when an incipient or actual skid or a locked wheel condition (all hereinafter being referred to collectively as skid) exists. Upon detecting a skid, the system dumps part or all (hereinafter a percentage) of the brake operating fluid pressure (hereinafter air pressure) which the vehicle operator then may be attempting to apply manually, e.g. by pressing on the vehicle brake pedal with his foot. The wheels then spin up to regain traction after which full requested brake pressure is reapplied, with such operation reducing skids and their undesirable results.
In the system disclosed in the '535 application, each of plural wheels is monitored by respective transducers that detect the rotational wheel speeds and produce respective AC transducer signals having frequencies representing wheel speed. Respective frequency to voltage converters produce DC voltages having amplitudes indicative of the AC transducer signal frequencies, thus being representative of wheel speed. Moreover, a single deceleration differentiator detector responds to downward-going changes in the smaller one of the two DC voltages to produce a deceleration signal proportionally representative and indicative of the rate of deceleration of the corresponding wheel. A wheel lock detector is provided for producing an output signal causing the dumping of a fixed amount of the air pressure when a locked wheel condition occurs. A single acceleration differentiator detector responds to upward-going changes in the DC voltage indicative of the slower wheel to produce an acceleration signal proportionally representative and indicative of the spin up rate or acceleration rate of that wheel. The purpose of the acceleration detector is, after a skid condition has existed, to avoid the reapplication of full air pressure if the wheels are accelerating too rapidly, for such full reapplication could cause a further skid to occur.
However, the various prior acceleration detectors used in skid control systems were not adaptive to the coefficient of friction between the road surface and tires, for example, and, accordingly, did not vary the percentage of reapplied air pressure according thereto.
It would be desirable to provide control of the reapplied air pressure according to the vehicle wheel acceleration rate.
Further, in the '535 application the advantages of effecting skid control operation in response to the more slowly rotating one of plural vehicle wheels is disclosed. For the same reasons, it would be desirable to effect the above acceleration rate responsive control of reapplied pressure in response to the more slowly rotating one of plural wheels.
Dynamic load transfer phenomenon is the change in loading of respective vehicle wheels in response to braking operation or deceleration. It has been found that the amount of frictional force between vehicle tires or wheels and a road surface, for example, to effect a braking or stopping function of the vehicle, will vary according both to the coefficient of friction (mu) between the road surface and tires and to the results of such dynamic load transfer phenomenon. It would be desirable to effect a skid control function that is adaptive to the mu condition and to the results of dynamic load transfer phenomenon, thereby to maximize braking efficiency.
Also disclosed in the '535 application is an output control circuit responsive, for example, to the magnitude of the deceleration signal to control the percentage of the requested air pressure that is in fact applied to the brakes. As is described, an air brake valve, such as one disclosed in a brochure entitled "Triple Action Skid Control", published by the B. F. Goodrich Company, March 1975, has a pair of selectively energized solenoids in a modulator valve portion which control the dumping of a percentage of the air pressure being requested by the driver. The output control circuit has four operational steps. In a step 0 operation, neither solenoid is energized, and 100% of the requested air pressure is applied to the vehicle brakes. In a step 1 operation, the output control circuit energizes one solenoid to dump about 33% of the requested air pressure, whereby approximately 67% of the requested air pressure is applied to the brakes. Similarly, in a step 2 mode, the output control circuit deenergizes the first and energizes the second solenoid to dump about 67% of the requested air pressure, thereby permitting application of about 33%; and in a step 3 mode, the output control circuit energizes both solenoids to dump 100% of the air pressure then being requested by the driver. The output control circuit also responds to a locked wheel signal from the wheel lock detector to go into a step 3 mode and, in addition, responds to production of an acceleration signal by the acceleration differentiator detector ordinarily to go into a step 2 mode.
The foregoing is, of course, described for exemplary purposes only, it being appreciated that other parameters than those mentioned may be used in the invention disclosed in the '535 application as well as in the present application.